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<a name="g_t_005f_005fhas_005fattribute-1"></a>
<h4 class="subsection">4.2.6 <code>__has_attribute</code></h4>
<a name="index-_005f_005fhas_005fattribute"></a>

<p>The special operator <code>__has_attribute (<var>operand</var>)</code> may be used
in &lsquo;<samp>#if</samp>&rsquo; and &lsquo;<samp>#elif</samp>&rsquo; expressions to test whether the attribute
referenced by its <var>operand</var> is recognized by GCC.  Using the operator
in other contexts is not valid.  In C code, <var>operand</var> must be
a valid identifier.  In C++ code, <var>operand</var> may be optionally
introduced by the <code><var>attribute-scope</var>::</code> prefix.
The <var>attribute-scope</var> prefix identifies the &ldquo;namespace&rdquo; within
which the attribute is recognized.  The scope of GCC attributes is
&lsquo;<samp>gnu</samp>&rsquo; or &lsquo;<samp>__gnu__</samp>&rsquo;.  The <code>__has_attribute</code> operator by
itself, without any <var>operand</var> or parentheses, acts as a predefined
macro so that support for it can be tested in portable code.  Thus,
the recommended use of the operator is as follows:
</p>
<div class="smallexample">
<pre class="smallexample">#if defined __has_attribute
#  if __has_attribute (nonnull)
#    define ATTR_NONNULL __attribute__ ((nonnull))
#  endif
#endif
</pre></div>

<p>The first &lsquo;<samp>#if</samp>&rsquo; test succeeds only when the operator is supported
by the version of GCC (or another compiler) being used.  Only when that
test succeeds is it valid to use <code>__has_attribute</code> as a preprocessor
operator.  As a result, combining the two tests into a single expression as
shown below would only be valid with a compiler that supports the operator
but not with others that don&rsquo;t.
</p>
<div class="smallexample">
<pre class="smallexample">#if defined __has_attribute &amp;&amp; __has_attribute (nonnull)   /* not portable */
&hellip;
#endif
</pre></div>




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